As has been observed in the aforementioned copending application, nuclear electricity-generating plants from time to time must dispose of radioactive waste water which contains dissolved solids and particularly boric acid. It is known, as described in that application, to reduce the quantity of radioactive material which must be stored, by concentrating the waste water, i.e. evaporating H.sub.2 O therefrom and thereby increasing the boric acid concentration to a level which is greater than that before the concentration step so undertaken and greater than the saturation concentration at room temperature, the liquid then being cooled to precipitate out the boric acid. The precipitated solids can be separated from the supernatant liquid by decantation and the liquid can be recycled to the evaporator in which the concentration step is effected.
The evaporation is thereby carried out to a boric acid concentration which is less than the storage concentration at the evaporation temperature, but more than the storage concentration at room temperature, thereby ensuring a significant degree of solids precipitation and sedimentation on cooling to room temperature.
The apparatus for carrying out this process thus comprises an evaporator whose input side receives the waste water fed from the nuclear reactor or recycled waste water from the decantation step, and a storage vessel connected to the output of the evaporator and in which the cooling can be carried out to sediment out the solids, namely the boric acid. The outlet of this storage vessel communicates with the inlet to the evaporator for recycling the decantate and a solids or slurry outlet can be provided at the bottom of this vessel so that the solids can be discharged.
The waste water which is used in the earlier system and for which the present method has been found to be particularly effective, can contain up to about 4,000 parts per million of boron, which is equivalent to 2.5 weight percent of boric acid as solids.
In this earlier system, the solids were generally embedded in a matrix of concrete, bitumen or synthetic resin directly upon withdrawal from the storage vessel or after storage.
Because of the large quantities of the sedimented material which must be stored in many cases, the storage capacity of the plant may be readily exhausted. This, of course, poses a major problem.